The present invention relates to the field of communications and more particularly to systems and methods for satellite communications.
Mobile communications systems allow users access to a communications network through respective user terminals as the user moves within a geographic region. Some mobile communications systems partition the geographic region into areas wherein each area may be covered by a portion of the mobile communications system. Consequently, if a user moves from a first area to a second area within the serviced geographic region, the mobile communications system may stop providing service in the first area and start providing the service in the second area to maintain the user""s access. The users may access the mobile communications system using a user terminal.
FIG. 1A illustrates a satellite communications system which provides communication service to user terminals located within areas 120 and 125 via respective spot beams 101 and 102. The satellite system provides communication service to user terminal 100 by registering the user terminal 100 within the spot beam corresponding to the area within which the user terminal is located. For example, if the user terminal 100 were located in area 125, the satellite system would register the user terminal 100 as located in area 125 and communicate with the user terminal 100 via spot beam 102. If, however, the user terminal 100 were to relocate to area 120, the satellite system may re-register the user terminal 100 as located in area 120 and subsequently communicate with the user terminal 100 via spot beam 101.
Spot beams 101 and 102 may provide respective communications channels for communications between the satellite 110 and the user terminal 100. In particular, a spot beam may provide respective control channels to user terminals within the area covered by the spot beam. A control channel may carry information concerning the service provided to the area covered by the respective spot beam. For example, spot beam 102 may broadcast information over a control channel which identifies spot beam 102 to user terminals located within area 125, and may also identify traffic channels assigned to the spot beam. Spot beams may also be grouped in beam pairs so as to identify spot beams which are adjacent. For example, spot beam 102 and spot beam 101 form a beam pair 115. Moreover, a user terminal 100 may be registered with spot beam 101, spot beam 102, or the beam pair 115.
The user terminal 100 may be a communications device, such as a radio telephone, which is capable of communicating with the satellite communications system.
The location of the user terminal 100 may be registered via a location update procedure. The location update procedure is a communications protocol whereby the user terminal 100 may notify the satellite system of the user terminal""s location within the geographic region. A location update procedure may be performed when the user terminal 100 moves to a new area and requires re-registration in the new area. The user terminal 100 may initiate the location update procedure when the user terminal 100 detects that the signal strength of its present spot beam is less powerful than that of another spot beam. For example, if the user terminal 100 were located within area 125 serviced by spot beam 102 and subsequently moved to area 120, the user terminal 100 may detect the greater signal strength of spot beam 101. Upon detecting the greater signal strength of the spot beam 101, the user terminal 100 switches from the control channel associated with spot beam 102 to the control channel associated with spot beam 101 and notifies the satellite communications system of its new location corresponding to area 120 using the information broadcast on the control channel associated with spot beam 101. Thus the user terminal 100 re-registers with the satellite system in area 120. Subsequently, the satellite communications system communicates with the user terminal 100 using spot beam 101. The user terminal 100 may also perform periodic location update procedures if the user terminal 100 operates in a particular location for some period of time.
Some satellites may exhibit a behavior known as an inclined orbit wherein the spot beams projected onto the geographic region periodically shift or oscillate. Such an oscillation is illustrated in FIG. 1B. At a time t1, the satellite 110 services the area 120 via spot beam 101 and the area 125 via spot beam 102. The user terminal 100 is located within the area 125 serviced by spot beam 102. At a time t2, the inclined orbit may cause an oscillation in the spot beams covering areas 120 and 125. Specifically, spot beam 101, formerly covering area 120, now covers area 120xe2x80x2. Similarly, the spot beam 102, formerly covering area 125, now covers area 125xe2x80x2. Moreover, user terminal 100, formerly located in area 125, is now located within the area 120xe2x80x2 serviced by spot beam 101. Thus, the user terminal 100 has experienced a shift in spot beam service without moving. At a time t3, the coverage shown at time t1 may be restored, causing yet another shift in the coverage of the spot beams and the service to the user terminal 100. The oscillation associated with an inclined orbit may be such that the shift in spot beam coverage described in FIG. 1B happens periodically.
As described above, the user terminal 100 may initiate location update procedures upon detecting a change in the spot beam service. At time t1, for example, the user terminal 100 detects service via spot beam 102. At time t2, however, user terminal 100 detects a change such that its service is provided by spot beam 101. Consequently, the user terminal 100 may initiate a location update procedure. Subsequently, the user terminal 100 may detect another shift in spot beam service when the inclined orbit causes an oscillation in the reverse direction. Consequently, the user terminal may perform yet another location update. Moreover, a location update procedure may be requested for each periodic shift in the spot beam coverage.
The user terminal 100 may also detect a change in spot beam service when the user terminal moves from one area to another. For example, if user terminal 100 were moved from area 125 to area 120, the user terminal may detect the change in spot beam service from spot beam 102 to spot beam 101. The user terminal 100 may then perform a location update procedure to register with spot beam 101.
Existing systems may use a registration process to reduce the number of location updates produced by oscillations in spotbeam coverage by presuming that the original spot beam coverage will resume within a predetermined time. The registration process may require each spot beam to broadcast a single location area code (LAC) which uniquely identifies the spot beam within the satellite system and a list of neighboring spot beams called beam pairs on a corresponding control channel. When the user terminal 100 detects a shift in its spot beam service, the user terminal 100 may determine whether a location update procedure is required or if a location update may possibly be unnecessary due to the oscillation behavior described above. The information broadcast on the control channel may require considerable system bandwidth for each spot beam.
If the user terminal 100 determines that the new servicing spot beam forms a beam pair with the registered spot beam, the user terminal 100 may register with the spot beam pair by performing a location update procedure so as to avoid repeatedly re-registering with a new spot beam every time the oscillation occurs. When the user terminal 100 registers with the servicing beam pair, the user terminal 100 starts a beam pair location update timer, wherein the duration of the beam pair location update timer exceeds the period of oscillation associated with the inclined orbit. For example, if the inclined orbit were such that the period of oscillation were six hours, the duration of the beam pair location update timer 220 would be greater than six hours. If the user terminal 100 detects the restoration of the original spot beam coverage before the expiration of the beam pair location update timer, the user terminal 100 stays registered with the spot beam pair currently providing service.
If, however, the user terminal 100 does not detect the restoration of the original spot beam coverage before the expiration of the beam pair location update timer, the user terminal 100 will re-register with the single servicing spot beam by performing another location update procedure. Consequently, if a shift in spot beam coverage is due to the movement of the user terminal 100 and not due to the inclined orbit behavior described above, the user terminal 100 will perform two location updates to complete the registration with a single spot beam: first when presuming oscillating coverage and second when the beam pair location update timer expires. Consequently, systems according to the prior art may perform redundant location updates when the user terminal is moved from a first spot beam to an adjacent spot beam that defines a spot beam pair with the first spot beam.
Errors which go undetected during the implementation of a satellite communications system such as those described herein, may be costly to repair. For example, a design error may be expensive or complex to fix if each user terminal 100 needs to be serviced. Furthermore, operational adjustments of the satellite communications system may be difficult for similar reasons. For example, changes in the inclined orbit behavior may be difficult to accommodate without altering the individual user terminals.
Notwithstanding the communications systems and methods discussed above, there continues to be a need to further reduce the number of location update procedures which occur within satellite communications systems and allow a reduction in the cost and complexity of addressing system errors and implementing system adjustments.
It is therefore an object of the present invention to provide improved satellite communications systems and related methods and terminals.
It is another object of the present invention to reduce the number of location update procedures within a satellite communications system.
It is still another object of the present invention to reduce the complexity of user terminals used within a satellite communications system.
These and other objects are provided by methods of registering a user terminal at a satellite communications system including a plurality of satellite spot beam transceivers, wherein each of the satellite spot beam transceivers defines a respective spot beam. The current spot beam registration information is transmitted from the user terminal to the satellite communications system. Updated spot beam registration information is determined for the user terminal at the satellite communications system based on the current spot beam registration information received from the user terminal. Updated spot beam registration information is transmitted from the satellite communications system to the user terminal. The user terminal updates the current spot beam registration information for the user terminal using the updated spot beam registration information received from the satellite communications system.
In particular, the current spot beam registration information may include a user terminal timer value which provides a duration of service provided by the current servicing spot beam within a spot beam pair. The current spot beam registration information may also include a current servicing spot beam and a previous servicing spot beam wherein the current servicing spot beam currently provides service to the user terminal and wherein the previous spot beam provided service to the user terminal immediately prior to the current servicing spot beam.
The present invention offers several advantages over the prior art. In particular, determining updated spot beam registration at the satellite communications system may help reduce the cost and complexity of correcting design errors and implementing operational adjustments. Instead, operational adjustments and corrections may be implemented at the satellite communications system rather than at each user terminal. In addition, the present invention may allow a reduction in the number of location update procedures associated with user terminal mobility.
In particular, the user terminal can use one location update procedure to register with a single spot beam after having been registered with a spot beam pair. This situation may be common when a user terminal is moved from a single spot beam to an adjacent spot beam which defines a spot beam pair with the previous servicing spot beam. In contrast, according to the prior art, the user terminal may initiate an additional location update procedure to register with a single spot beam after having been registered with a spot beam pair. Consequently, moving the user terminal to an adjacent spot beam may cause an additional location update procedure to be used according to the prior art.
The transmission of the current spot beam registration information to the satellite communications system is triggered by communication between the user terminal and the satellite communications system. In particular, the user terminal timer value is transmitted to the satellite communications system in conjunction with communications, such as a telephone call. The satellite communications system can thereby monitor the duration of the current servicing spot beam within a spot beam pair. Upon detecting that the timer value exceeds the oscillation period associated with the inclined satellite orbit, the satellite communications system updates the registration of the user terminal to the single current servicing spot beam. Thus, the present invention can reduce the number of location update procedures used to register the user terminal with a single spot beam when the user terminal is moved to an adjacent spot beam which defines a spot beam pair with the previous servicing spot beam. In addition, the present invention allows the registration process described herein to be modified at the satellite communications system after implementation of the system. Alternately, the registration process may be disabled when the oscillating behavior associated with the inclined orbit ceases or is diminished.
The transmission of the current spot beam registration information to the satellite communications system is also triggered by detection of new servicing spot beam other than the current servicing spot beam and the previous servicing spot beam and the expiration of a periodic location update timer wherein the periodic location update timer measures a duration of service provided to the user terminal since a previous expiration of the periodic location update timer. The present invention uses the triggered location update procedures to transmit the current spot beam registration information to reduce the number of location updates used by the satellite communications system. The present invention therefore may allow a reduction in the satellite communications bandwidth by using fewer location update procedures to transmit information to the satellite communication system.